Lithium ion secondary batteries are widely used in various kinds of electronic devices and components, particularly in portable telephones, note-book personal computers, video cameras, satellites, electric motorcars, etc.
Among lithium ion secondary batteries, lithium ion polymer secondary batteries use a gel electrolyte containing a conductive polymer or the like as electrolyte. Compared to a lithium ion battery that uses a nonaqueous electrolyte, a lithium ion polymer secondary battery is highly safe because the possibility of leakage of the electrolysis solution is low, and the lithium ion polymer secondary battery can be made smaller in size and lighter in weight. Therefore, the lithium ion polymer secondary battery is one of the batteries that is expected to meet its increasing market demand.
From the viewpoint of productivity, quality stability and the like, as an encasement for lithium ion polymer secondary batteries, a metallic can of cylindrical shape, a rectangular parallelepiped shape or the like, that is obtained by press molding a metal plate, metal foil and the like is generally used.
However, when a metal can is used as an encasement for a secondary battery, the shape and design of the battery itself are subject to many restrictions and the electronic device or component that is equipped with the battery with a metal can encasement is also subject to restrictions on the shape and design of the part in which the battery is stored. This leads to the problem that the electronic devices and components cannot be formed into a desirable configuration, making it difficult to render the electronic devices and components further miniaturized and lighter.
Therefore, in order to form an electronic device and component into a desired configuration and achieve further miniaturization and lightening in weight thereof, development of a battery encasement that can be readily formed into a shape that fits the shape of the electronic device or electronic component has been desired. As a metal to be used for battery encasement, aluminum foil is attracting widespread attention.
A laminate for battery encasement for use in a lithium ion polymer secondary battery should have the properties that meet the following requirements.
(1) The laminate should have gas barrier properties that can insulate the principal part and the electrodes in the main body of the secondary battery from outside air (particularly containing water vapor) in order to avoid the undesirable phenomenon of hydrofluoric acid generated by hydrolysis of the electrolyte corroding the aluminum foil, when water vapor enters into the encasement for a secondary battery from outside.
(2) The innermost layer of the laminate should exhibit excellent adhesiveness to the metal electrodes that compose the secondary battery and excellent adhesiveness between the innermost layers.
(3) The secondary battery should have properties (heat resistance and cold resistance) resistive to the temperatures at which the battery is used, specifically, the temperature in an automobile during the summertime, in a cold district during the wintertime, etc. The temperature in the secondary battery is further increased by charging or discharging during use of the secondary battery. Even when the secondary battery is used under these severe circumstances, as a battery encasement, the laminate should have stable thermal adhesiveness, gas barrier properties, etc.
(4) The adhesive strength between layers should not be weakened by the gel electrolyte (gelled electrolyte solution) that is used in the secondary battery.
(5) The laminate for battery encasement should be resistant to corrosion by hydrofluoric acid generated by deterioration, hydrolysis, or the like of the gel electrolyte used in the secondary battery.
(6) The laminate for battery encasement should have the drawability and the like to be easily shaped into a desirable shape, and be easy to produce.
However, a laminate for battery encasement having such excellent properties has not yet been developed.